According to the known prior art, IOLs are selected or adapted based on measured and/or estimated variables, only individual parameters in the form of individual measured values or as a mean for a defined patient group being taken into account.
The optimal intraocular lens (IOL) is selected or adapted exclusively according to its features, such as for instance type, refractive power, asphericity, and multifocality. No consideration is given to potential interrelationships with specific contributing factors to the treatment, such as patient features, diagnostics, surgical procedures, or the like, or to using statistical distribution for the parameters.
Cataract surgeons are required to select suitable intraocular lenses (IOL) for a patient. The surgeon must account for many factors. Firstly, the suitable calculation method for the IOL optical power must be selected. For this, as a rule different more or less suitable formulas must be used for the calculation for unusually long, normal, or unusually short eyes. In the simplest cases, the input parameters for these calculations are based on the keratometry and axial length of the eye. Due to their simplified model assumptions, the formulas generally also include an empirically determined correction factor, such as for instance the so-called A constant.
The currently most widely used calculation methods are so-called IOL formulas, e.g. Holladay, Hoffer, Binkhorst, Colenbrander, Shammas, and SRK IOL formulas. According to these, the refraction D (starting/evaluation parameter) of the patient after IOL insertion is calculated with:D=DIOL−f(K, AL, ACD, A)  (1)    where f( ) is a classically known IOL formula and            DIOL is the refractive power of the IOL,        K is the measured keratometry value,        AL is the measured axial length of the eye,        ACD is the measured anterior chamber depth, and        A is an IOL type-dependent constant input variable.        
For selecting the IOL, the doctor specifies a target refraction (D=DTARGET). For optimization, the doctor calculates the refraction in accordance with (1) for different IOLs by varying DIOL and A. In many cases the doctor uses IOLs of the same type so that there is no variation in A and the optimization amounts to a formula calculation according to DIOL=DTARGET+f(K, AL, ACD, A). Thus, if the target is emmetropia, the classic formula calculation for the IOL that results is DIOL=f(K, AL, ACD, A).
The constant A in the formulas is determined empirically using a patient ensemble in order to adapt the formula values to the actually resulting optimal refraction values. However, this adaptation only ensures that the mean of the refraction values for the test ensemble agrees with the formula.
The doctor typically accounts for statistical errors in the biometry formula in that he knows from experience that his actually attained refraction values for his patients will have a certain fluctuation around the target refraction. If he wants to minimize its effect, he provides a correction to the target refraction. For instance, if the doctor typically has deviations of +/−0.25 D for target refraction in patients with myopic eyes, then he will target refraction of −0.25 D in order to have a high probability of preventing the eye of the patient from being intolerably hyperopic. This method represents a good strategy for the average in the patient ensemble.
However, the typical fluctuation around the target refraction or the allowance could be reduced if, instead of a mean value for a patient ensemble, individual input parameters for the individual patient were to be used as the initial variables.
Currently various approaches are used according to the prior art in order to minimize systematic errors.
Thus, a number of doctors use a different. A constant for each ethnic group among their patients. This permits a reduction in the systematic errors and also permits a reduction in statistical errors if the statistical variance in each group is lower.
Depending on defined starting conditions such as for instance patients having long axial eye lengths or that have had previous refractive corneal surgery, other doctors use different biometric formulas that are better adapted to the conditions in a specific case or that require measurement of additional parameters, such as the anterior chamber depth or lens thickness. In this case, as well, in particular the systematic errors are reduced, but statistical errors may increase due in part to the additional measured parameters.
Thus for instance U.S. Pat. No. 5,968,095A describes a method for pre-operative selection of an intraocular lens in which it is to be assured that the eye has a desired post-operative refractive power. This is to be attained in that the location of the lens haptic plane, the corneal refractive power of the eye, and the axial length of the eye are determined and the desired post-operative refractive power is selected. For an IOL to be implanted, the refractive power and geometry of which are known, an offset between the lens haptic plane and the anterior vertex of the lens is prespecified as if it were in its implanted condition. Then a calculation is made to check whether the focus of the selected IOL, with the aforesaid specifications and the refraction indices of the ocular fluids, will fall post-operatively onto the retina of the eye. If this is not the case, the calculation is performed over again for a different IOL having a different refractive power and/or geometry. For the implantation, an available IOL of nearest refractive power for which focusing on the retina has been calculated is selected for the implantation.
An alternative method, albeit a method that is not widely used, is ray tracing. As the term indicates, ray tracing shall be construed as a method for tracing/following rays. As is known, we only perceive objects in our environment because they are irradiated by a light source and they reflect these rays of light, some of which ultimately reach our eyes. The ray tracing method simulates this elementary natural phenomenon. If the optical system, i.e., the individual human eye with all of its optical elements, is known, a “real” image occurring on the retina may be calculated by means of ray tracing. The method is thus based on a detailed eye model using the corneal topography of the eye. In this method, no general correction factors (A constants) are used, but certain assumptions regarding the effective (post-operative) lens position (ELP) must be made. This method is suitable for eyes having widely varying biometric parameters, such as for instance long eyes, normal eyes, short eyes, post-LASIK eyes, etc.
The IOL optical power and the residual refraction are then calculated using ray tracing. Various selection criteria and metrics for the calculation may be used in order to attain a good correlation to subjective visual acuity, i.e. a result comparable to what the patient experiences. Although retinal image metrics have proved to be particularly suitable, the following other selection criteria are also possible:                Evaluation of the image on the retina with respect to moment, entropy, compactness, shape, and intensity distribution by means of point spread function (PSF), line spread function (LSF), and root mean square;        Evaluation of resolution using optical transfer function (OTF), such as modulation transfer function (MTF) or phase transfer function (PTF);        Evaluation of contrast using the contrast sensitivity function (CSF);        Evaluation of optical aberrations, such as chromatic aberration, ray aberration, wavefront aberration, depth of field, and binocular deviation of the image scale;        Evaluation of the classic refraction parameters: diopter and astigmatism.        
This list merely provide examples, because in principle other optical evaluation parameters known to one skilled in the art may also be used. In addition, in principle any evaluation parameters or criteria with which deviations from the ideal wavefront may be assessed and quantified may be used.
U.S. Pat. No. 7,357,509 B2 describes some metrics that are particularly suitable for predicting the subjective impacts of wavefront aberrations of an eye. The metrics used may be based on the effective values or the increase in wavefront errors, the area of the critical pupil, a curvature parameter, the point spread function, the optical transfer function, or the like.
While P.-R. Preussner et al [1] compare the use of ray tracing methods and IOL formulas, publication [2] goes into more detail regarding a calculation model that is based on a method of ray tracing. In this case, based on the individual measured values and estimated variables such as especially the position of the IOL in the eye, an eye model with as a rule a plurality of optically active surfaces is developed and is calculated for one or a plurality of rays using methods from the optical design. The image quality on the retina/fovea is calculated as the evaluation value. With appropriately precise determination of the input variables this makes it possible to avoid systematic errors to a large extent. Statistical errors that result for example from lack of reproducibility of measurements or from fluctuations in the wound healing process are not taken into account here, either.
J. Einighammer et al describe another method for calculating the exact geometry of customized IOLs for pseudophakic eyes that is based on ray tracing in [3]. An individual calculation model is designed using measurements. During the optimization process, which includes the geometry of the customized IOL, so-called real ray tracing is used to try to obtain the minimum of wavefront errors.
In [4], L. N. Thibos et al investigate the extent to which the use of different metrics, such as for instance pupil plane and image plane metrics, impact the accuracy and precision of predicting the results of wavefront aberrations. It was found that there are certainly differences in the precision of predictions, but that the accuracy of all methods may be improved by correcting systematic bias.
In addition to the IOL optical power, certain parameters, such as asphericity and toricity or the cornea, provide indications for certain IOLs. In the case of so-called premium IOLs, after consulting with the patient, the surgeon may decide in favor of IOLs that satisfy specific visual tasks, such as e.g. multi-focal lenses. Such IOLs should make it possible for the patient to perform visual tasks in the near range and far range without additional vision aids. How the IOLs used in the individual eyes actually satisfy the requirements imposed on them is a function of a number of factors, such as for instance the optics of the cornea, the implantation technique, the optical and mechanical design of the IOL, the pathologies of the eyes, etc.